1. Field of the Invention
This invention relates generally to circuit breakers, and particularly to a two-pole miniature circuit breaker provided with an arrangement for calibrating the delayed trip function with the circuit breaker fully assembled.
2. Background Information
Circuit breakers designed for residential and light commercial applications are typically referred to as miniature circuit breakers. Such circuit breakers have pole mechanisms which include separable contacts, a spring powered operating mechanism with a handle for manual opening and closing of the separable contacts and a trip assembly for automatically opening the separable contacts. The trip assembly includes a bimetal providing a thermal or delayed trip in response to a persistent overcurrent condition, and a magnetic armature providing a magnetic or instantaneous trip in response to a higher level overcurrent. The pole mechanism is mounted in a housing molding of an insulative resin and having a compartment in which the pole mechanism is assembled. A molded cover is then secured in place to enclose the pole mechanism.
Industry standards require that the thermal trip device in these circuit breakers be calibrated to trip the breaker in response to an overcurrent of a predetermined magnitude within a specified time interval. Traditionally, this calibration of the thermal trip is performed "on the half shell". That is, the pole mechanism is assembled within the compartment of the molded housing, and the thermal trip is calibrated before the mechanism is enclosed by the cover.
A common type of circuit breaker in which the thermal trip is calibrated in this manner is shown by way of example in U.S. Pat. No. 3,849,747. Such circuit breakers have been used for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured on a large scale. This type of circuit breaker has a metal frame or support plate with an integral tab extending laterally from one end to which the bimetal of the thermal trip device is secured. The end of the support plate from which the tab extends is partially separated by a transverse slot from the remainder of the support plate which is fixed in the housing. The bimetal is calibrated by closing the circuit breaker and applying the prescribed overcurrent. A tool is inserted in the transverse slot in the support plate and when a specified time has expired, the tool is rotated to distort the free end of the support plate thereby adjusting the position of the support for the bimetal to cause the bimetal to trip the breaker. This calibration has traditionally been carried out automatically, "on the half shell" by a machine. With the calibration set, the cover is installed and riveted in place. The circuit breaker is then tested to validate the calibration. Circuit breakers which do not pass the calibration test are reworked by inserting a hook through a slot in the end of the circuit breaker to engage the free end of the bimetal to attempt to bring it within tolerance. Such reworking is done manually, and being difficult to perform only results in bringing about half of the rejected circuit breakers into tolerance.
It has been determined that the number of circuit breakers which fail the calibration test performed after the cover has been installed is due in part to minor changes in position and distortion of the mechanism resulting from the misalignment of the housing parts causing the breaker to fall out of calibration. In order to overcome these effects, U.S. Pat. No. 4,148,004 proposes a single pole circuit breaker of this type which is fully assembled with the cover riveted in place, and then calibrated by a plug rotatably mounted in the wall of the housing and having a bifurcated stem which engages the tab on the support plate carrying the fixed end of the bimetal. A tool inserted in apertures in the external face of the calibrating plug is rotated to set the calibration. Thus, the circuit breaker is calibrated after it is fully assembled and the parts are fixed in their final position. However, it also allows one to change the calibration which is not in conformance with electrical codes in the United States.
U.S. Pat. No. 5,008,645 discloses a single pole miniature circuit breaker which can be calibrated after full assembly and which provides an indication of any subsequent tampering with that calibration. This patent provides an opening in the housing which is aligned with the slot at the interface between the fixed portion of the metal support frame and the free end to which the bimetal is fixed. The automatic tool is inserted through this opening in the housing and rotated to bend the free end of the support plate relative to the fixed end to thereby adjust the trip point of the bimetal. The calibration is made tamper evident by applying a seal over the calibration opening once the thermal trip has been calibrated.
Both the U.S. Pat. Nos. 4,148,004 and 5,008,645 are directed to single pole miniature circuit breakers. Two-pole miniature circuit breakers are also available. These circuit breakers have identical pole mechanisms housed in side-by-side compartments within the molded housing. Such circuit breakers are commonly used in circuits where both pole mechanisms must be either open or closed. Thus, the handles are tied together so that the two poles are simultaneously opened and closed manually. A common trip device assures that when one pole mechanism trips, the other is tripped also. The common trip device includes actuating members in each pole compartment keyed on a common shaft so that when one actuating member is rotated the other is rotated also. The actuating members include a nose which is engaged by the tripping of a pole mechanism. Each of the actuating members also includes a finger which engages and trips the magnetic actuator of the associated pole when the common trip device is rotated. Thus, when either pole mechanism trips, either thermally or magnetically, the other pole is also tripped.
The metal support plate of one the poles of the two-pole miniature circuit breaker is adjacent to the outer wall of the associated pole compartment, and therefore, can be calibrated in the assembled state in the same manner as the single pole breaker of U.S. Pat. No. 5,008,645. That is, the tool can be inserted through an opening in the housing wall in direct alignment with the calibration slot in the metal support plate. However, since the two-pole mechanisms are identical, the metal support plate of the other pole is not adjacent an outer wall of the housing, but is separated from it by the remainder of the pole mechanism. More of a problem however, is that the actuating member of the common trip device interferes with the insertion of a calibrating tool into the calibration slot of this pole mechanism when the two-pole breaker is assembled. Thus, heretofore it has not been possible to calibrate the two-pole miniature circuit breaker while fully assembled. Consequently, the two-pole circuit breakers have continued to be calibrated "on the half shell" with all of the attendant problems discussed above, which are compounded by the need to achieve proper calibration of both poles.
There is a need therefor for an improved two-pole circuit breaker which can be calibrated when fully assembled.